1. Field of the Invention
The present invention relates to shock sensors and more particularly to a shock sensor for use in an air bag system for automobiles.
2. Description of Related Art
An air bag system is employed in an automobile in order to protect the driver from a shock when the automobile collides with another automobile or the like. A shock sensor detects a shock and outputs a detection signal. The detection signal activates a firing device of an air bag which in turn causes the air bag to quickly inflate to protect the driver from the shock.
Prior art shock sensors are disclosed in, for example, Japanese Patent Preliminary Publications No. 7-198738 and No. 8-29444.
FIG. 16 illustrates a conventional shock sensor.
Referring to FIG. 16, a magnet 16 is housed in a case 11 and attached to one end of a shock-sensing element 15 which is swingably supported on a pin 14. The shock-sensing element 15 is normally at its neutral position shown in FIG. 16 so that the magnet 16 is normally positioned directly over a magnetic element 17 fixedly mounted in the case 11. The neutral position is a position where the shock-sensing element 15 is at rest when no shock is applied thereto. Mounted on both sides of the magnetic element 17 are reed switches 12 and 13 which are closed when the magnet 16 moves to positions over the reed switches 12 and 13. The reed switches are normally open since the magnet 16 is directly over the magnetic element 17.
If a shock is applied to the shock sensor in a direction shown by arrow P, the shock-sensing element 15 acquires a momentum and swings counterclockwise. Thus, the shock-sensing element 15 is moved out of its neutral position. Thus, the magnet 16 moves into an area where the reed switch 13 is closed.
However, the principle of the conventional art reed switch type shock sensor dictates that the sensor is mounted in such a way that the shock-sensing element 15 is always suspended vertically. In addition, the sensor detects shocks only in a narrow range of direction in which the shock-sensing element 15 can swing sufficiently. Thus, shocks in any other directions outside of the range are not detected properly. The shock is converted into an electrical signal in terms of on-off operation of the reed switches 12 and 13. Use of a reed switch is an obstacle to miniaturizing a shock sensor. Another problem with the conventional art sensor is that the detection signal of the shock sensor is outputted only in limited signal form since the reed switches in FIG. 16 are normally open.